Technology update

Aug 18, 2005

Carbon-nanotube fabric measures up

Researchers at the University of Texas at Dallas, US, and Commonwealth Scientific and Industrial Research Organization (CSIRO) Textile and Fibre Technology, Australia have drawn multiwalled carbon nanotubes into transparent sheets that are 5 cm wide and 1 m long. An array of the sheets had a higher strength-per-unit weight than high-strength steel.

And the sheets weren't just strong - the team also demonstrated their use as polarized-radiation sources, flexible organic light-emitting diodes, transparent elastomeric electrodes, conducting appliqués and in the microwave bonding of plastics.

"Rarely is a processing advance so elegantly simple that rapid commercialization seems possible, and rarely does such an advance so quickly enable diverse application demonstrations," said Ray Baughman of the University of Texas at Dallas. "Synergistic aspects of our nanotube sheet and twisted yarn fabrication technologies will likely help accelerate the commercialization of both technologies and the University of Texas at Dallas and CSIRO are working together with companies and government laboratories to bring both technologies to the marketplace."

The researchers drew the sheets from the sidewall of a "forest" of multiwalled carbon nanotubes synthesized by catalytic chemical-vapour deposition. The nanotubes were about 10 nm in diameter.

The researchers used forest heights of 70-300 μm - they found that the higher forests were generally easiest to draw into sheets. They also produced thicker sheets.

A 1 cm length of 245 μm-high forest, for example, produced a 3 m long freestanding nanotube sheet that was about 18 μm thick. By winding the sheet onto a rotating plastic cylinder the researchers were able to increase the production rate to 10 m/min.

The nanotube sheet produced was an electronically conducting, anisotropic aerogel with a density of 0.0015 g/cm3. The sheets could support millimetre-sized liquid droplets that were 50,000 times more massive than the sheet region in which they were in contact.

In order to create denser material, the researchers placed a sheet on a planar substrate and then vertically immersed it in ethanol along the direction of the nanotube alignment. Withdrawing the substrate caused ethanol evaporation. The associated surface-tension effects reduced the thickness of the sheets to around 50 nm and raised their density to roughly 0.5 g/cm3.

The densified sheets were both transparent and had good electrical conductivity - the scientists say that this combination is needed for applications such as displays, video recorders, solar cells and solid-state lighting. The sheet resistivity was about 700 Ω/m2 in the draw direction. The structures also retained most of their conductivity on bending, a feature that's vital for flexible electronic circuits.

The researchers found that they could also assemble the sheets into biaxially reinforced arrays. A densified stack of 18 nanotube sheets that were orthogonally oriented to their neighbours had a strength of 175 MPa/(g/cm3). This compares well to the Mylar and Kapton films used for ultralight air vehicles, which have a strength of about 160 MPa/(g/cm3), and ultra-high-strength steel at about 125 MPa/(g/cm3).

The sheet-drawing technique has the advantage that, unlike solution or melt-based processing methods, it works better for longer nanotubes. Longer nanotubes are more likely to produce a structure with good electrical and thermal conduction and better mechanical properties.